Think of it like a forward trade on the settlement price. If you are buying with a TAS you are agreeing to go long the futures contract at the settlement price (+/- the offset), and whoever you trade with is agreeing to go short at the same price. It is guaranteed because the exchange becomes the counterparty for both traders and there is a margin deposit.
You can cancel an order if it has not executed yet. There is a TAS order book where the prices are the offsets you are willing to trade at. Once your order has been matched, you are either long or short the TAS contract and have to trade out of it if you don't want to actually do the futures trade at the close, just as if you had a position in a futures contract and wanted to get flat before the delivery process starts.
TAS contracts get into the news periodically because they are used in a manipulation strategy called "banging the close" where traders take a position in TAS contracts then try to move the settle price in their favor with trades around the close. Here is an example: http://dodd-frank.com/second-recent-cftc-banging-the-close-case-results-in-various-cea-violations-fines-trading-bans/